I thought they were the Botany Bay's cargo containers - hence "SS Botany Bay" being written on the seat belt (?) strap.
Mike Okuda specifically stated that they were cargo containers from the Enterprise. They were just filled with the Botany Bay's stuff.
Now granted, the internal volume of the containers we saw in the movie seems to be a bit larger than your average cargo container:
...but that was probably fudged for the movie's sake. I remember a picture in Starlog magazine showing the cast and crew in front of the exterior of the cargo containers on Ceti Alpha V, and they looked like the above pic.